A typical MTJ etched by a RIE (reactive ion etching) process is found to have sidewall damage, possibly caused by oxygen or other chemicals during the RIE etching process. The sidewall damage will lead to a size dependence of magnetic resistance ratio (DRR) which will reduce the read margin for the MRAM (magneto-resistive random-access memory) chip functionality. One effective way to eliminate sidewall damage is the use of a pure physical etching process such as IBE (ion beam etching). According to our device data, the pure IBE process or RIE+IBE trimming process will eliminate the size dependence of MR (magneto-resistance) ratio. This means the sidewall damaged layer could be removed by an IBE or RIE+IBE process. However, one drawback of IBE or RIE+IBE is the sidewall redeposition from the bottom electrode when the IBE process is performed. The sidewall redeposition of the bottom electrode will lead to the shunting path around the MTJ sidewall and then lead to low yield for the MRAM chip.
Several patents show the use of IBE in removing damage. These include U.S. Pat. No. 8,728,333 (Wang et al) and US Patent Applications 2015/0104882 (Jung et al) and 2015/0044781 (Tokashiki). U.S. Pat. No. 8,691,596 (Nomachi) shows a sacrificial layer of silicon dioxide in an IBE process.